I find Nextcloud pretty great. There's a sync client available on f-droid. It also has capabilities for calendar and contact list synchronisation, so you can move that off your google account.

It came preinstalled with my instance of mail-in-a-box (https://mailinabox.email/), which is an easy way of hosting your own email. With my self-hosted instance of mailinabox and LineageOS my phone is completely google-free. :)

Edit: And it has a decent iOS sync client as far as I know.